is well known to use depletion mode FET circuits for operation at high frequencies, for example in radio frequency amplifiers and mixers. Such FET circuits typically comprise metal-semiconductor FETs (MESFETs) using semiconductors such as gallium arsenide (GaAs) or indium phosphide; however, the same comments and this invention can also be applied to other types of depletion mode FETs, including for example metal-oxide-semiconductor FETs (MOSFETs) and junction FETs. For various reasons, process variations which inevitably occur in the manufacture of such FETs and integrated circuits including such FETs result in varying characteristics of the manufactured devices.
For depletion mode FETs it has been recognized that enhanced and consistent RF operation can be established by controlling DC characteristics of the FETs, and in particular by controlling a bias voltage supplied to each FET. By providing an appropriate gate bias, a desired drain-source current of the FET can be determined and maintained, resulting in a predictable RF performance. Consequently, a useful yield of manufactured devices with relatively stable characteristics can be provided by a bias circuit which supplies to each FET a bias voltage to compensate for process variations in the manufacturing process. Accordingly, such a bias circuit is desirably provided as part of an integrated circuit including the FET to be biassed, so that the FET and the bias circuit are subject to the same process variations.
In addition, it is desirable for the bias circuit to provide compensation for other factors, such as aging and temperature variation, which can otherwise adversely affect the operating characteristics of the FET. Alternatively, it may be desired to provide a PET which has a predetermined non-zero dependence of one of its parameters, e.g. drain-source current, with a parameter such as temperature, for example to compensate for a similar but opposite dependence of other circuits with which the FET is used.
Thus there is a need to provide a bias circuit for a depletion mode FET which can facilitate achieving such desires.